The blades of a steam turbine erode during operation, particularly along the outer end of the leading edge, due to contact with solid particles and condensed water droplets entrained in the air flowing past the turbine blades. Because erosion is not uniform from one blade to the next it can eventually lead to an imbalance in the rotor.
A popular means for preventing turbine blade erosion involves fitting an erosion-resistant shield over the outer end of the leading edge of each blade. Typically the shield is fitted to the blade and brazed along its periphery by conventional brazing techniques such as induction heating. Over time, however, even the hardest materials will erode in the environment of the turbine, and thus such erosion shields must be replaced from time to time.
Removal of blades from the rotor is an expensive process resulting in significant down-time for the turbine, but with conventional techniques unless the rotor blade is removed so that a conventional induction heater can be utilized to debraze the shield, the erosion shield must be debrazed using an oxyacetylene welding torch. The uneven heating associated with manual debrazing by means of a welding torch can contribute to stress cracking susceptibility of the blade if the blade is hardened, thus reducing its useful life.
The present invention overcomes these disadvantages by providing a method and apparatus for the in situ debrazing of erosion shields utilizing induction heating rather than an open flame. The invention provides means for debrazing by induction heating and then detaching the erosion shield from the blade without removing the turbine blades from the rotor. The heating process is more controlled and uniform than processes involving an open flame, and better lends itself to automation and remote control by an operator. This not only reduces the cost and duration of the repair process, but also results in a better working environment for repair personnel.